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<h1 id="signals">Signals</h1>
<h2 id="signals-1">Signals</h2>
<p>Each object is encapsulated in Gtk programming. And it is not
recommended to use global variables because they are prone to make the
program complicated. So, we need something to communicate between
objects. There are two ways to do so.</p>
<ul>
<li>Functions. For example,
<code>tb = gtk_text_view_get_buffer (tv)</code>. The caller requests
<code>tv</code> to give <code>tb</code>, which is a GtkTextBuffer
instance connected to <code>tv</code>.</li>
<li>Signals. For example, <code>activate</code> signal on GApplication
object. When the application is activated, the signal is emitted. Then
the handler, which has been connected to the signal, is invoked.</li>
</ul>
<p>The caller of the function or the handler connected to the signal is
usually out of the object. One of the difference between these two is
that the object is active or passive. In functions the object passively
responds to the caller. In signals the object actively sends a signal to
the handler.</p>
<p>GObject signals are registered, connected and emitted.</p>
<ol type="1">
<li>Signals are registered with the object type on which they are
emitted. The registration is done usually when the object class is
initialized.</li>
<li>Signals are connected to handlers by <code>g_connect_signal</code>
or its family functions. The connection is usually done out of the
object.</li>
<li>When Signals are emitted, the connected handlers are invoked.
Signals are emitted on the instance of the object.</li>
</ol>
<h2 id="signal-registration">Signal registration</h2>
<p>In TfeTextView, two signals are registered.</p>
<ul>
<li>“change-file” signal. This signal is emitted when
<code>tv-&gt;file</code> is changed.</li>
<li>“open-response” signal. <code>tfe_text_view_open</code> function is
not able to return the status because it uses GtkFileChooserDialog. This
signal is emitted instead of the return value of the function.</li>
</ul>
<p>A static variable or array is used to store signal ID.</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">enum</span> <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>  CHANGE_FILE<span class="op">,</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>  OPEN_RESPONSE<span class="op">,</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>  NUMBER_OF_SIGNALS</span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a><span class="dt">static</span> guint tfe_text_view_signals<span class="op">[</span>NUMBER_OF_SIGNALS<span class="op">];</span></span></code></pre></div>
<p>Signals are registered in the class initialization function.</p>
<p>@@<span class="citation" data-cites="include">@include</span>
tfetextview/tfetextview.c tfe_text_view_class_init @@@</p>
<ul>
<li>6-15: Registers “change-file” signal. <code>g_signal_new</code>
function is used. The signal “change-file” has no default handler
(object method handler) so the offset (the line 9) is set to zero. You
usually don’t need a default handler. If you need it, use
<code>g_signal_new_class_handler</code> function instead of
<code>g_signal_new</code>. See <a
href="https://docs.gtk.org/gobject/func.signal_new_class_handler.html">GObject
API Reference</a> for further information.</li>
<li>The return value of <code>g_signal_new</code> is the signal id. The
type of signal id is guint, which is the same as unsigned int. It is
used in the function <code>g_signal_emit</code>.</li>
<li>16-26: Registers “open-response” signal. This signal has a
parameter.</li>
<li>24: Number of the parameters. “open-response” signal has one
parameter.</li>
<li>25: The type of the parameter. <code>G_TYPE_INT</code> is a type of
integer. Such fundamental types are described in <a
href="https://developer-old.gnome.org/gobject/stable/gobject-Type-Information.html">GObject
reference manual</a>.</li>
</ul>
<p>The handlers are declared as follows.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="co">/* &quot;change-file&quot; signal handler */</span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a>user_function <span class="op">(</span>TfeTextView <span class="op">*</span>tv<span class="op">,</span></span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a>               gpointer user_data<span class="op">)</span></span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a><span class="co">/* &quot;open-response&quot; signal handler */</span></span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span></span>
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a>user_function <span class="op">(</span>TfeTextView <span class="op">*</span>tv<span class="op">,</span></span>
<span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a>               TfeTextViewOpenResponseType response<span class="op">-</span>id<span class="op">,</span></span>
<span id="cb2-10"><a href="#cb2-10" aria-hidden="true" tabindex="-1"></a>               gpointer user_data<span class="op">)</span></span></code></pre></div>
<ul>
<li>Because “change-file” signal doesn’t have parameter, the handler’s
parameters are a TfeTextView instance and user data.</li>
<li>Because “open-response” signal has one parameter, the handler’s
parameters are a TfeTextView instance, the signal’s parameter and user
data.</li>
<li><code>tv</code> is the object instance on which the signal is
emitted.</li>
<li><code>user_data</code> comes from the fourth argument of
<code>g_signal_connect</code>.</li>
<li><code>parameter</code> comes from the fourth argument of
<code>g_signal_emit</code>.</li>
</ul>
<p>The values of the parameter is defined in <code>tfetextview.h</code>
because they are public.</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="co">/* &quot;open-response&quot; signal response */</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="kw">enum</span></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>  TFE_OPEN_RESPONSE_SUCCESS<span class="op">,</span></span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a>  TFE_OPEN_RESPONSE_CANCEL<span class="op">,</span></span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a>  TFE_OPEN_RESPONSE_ERROR</span>
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span></code></pre></div>
<ul>
<li>The parameter is set to <code>TFE_OPEN_RESPONSE_SUCCESS</code> when
<code>tfe_text_view_open</code> has successfully opened a file and read
it.</li>
<li>The parameter is set to <code>TFE_OPEN_RESPONSE_CANCEL</code> when
the user has canceled.</li>
<li>The parameter is set to <code>TFE_OPEN_RESPONSE_ERROR</code> when an
error has occurred.</li>
</ul>
<h2 id="signal-connection">Signal connection</h2>
<p>A signal and a handler are connected by the function
<code>g_signal_connect</code>. There are some similar functions like
<code>g_signal_connect_after</code>,
<code>g_signal_connect_swapped</code> and so on. However,
<code>g_signal_connect</code> is the most common. The signals
“change-file” is connected to a callback function out of the TfeTextView
object. In the same way, the signals “open-response” is connected to a
callback function out of the TfeTextView object. Those callback
functions are defined by users.</p>
<p>In the program <code>tfe</code>, callback functions are defined in
<code>tfenotebook.c</code>. And their names are
<code>file_changed</code> and <code>open_response</code>. They will be
explained later.</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>g_signal_connect <span class="op">(</span>GTK_TEXT_VIEW <span class="op">(</span>tv<span class="op">),</span> <span class="st">&quot;change-file&quot;</span><span class="op">,</span> G_CALLBACK <span class="op">(</span>file_changed<span class="op">),</span> nb<span class="op">);</span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a>g_signal_connect <span class="op">(</span>TFE_TEXT_VIEW <span class="op">(</span>tv<span class="op">),</span> <span class="st">&quot;open-response&quot;</span><span class="op">,</span> G_CALLBACK <span class="op">(</span>open_response<span class="op">),</span> nb<span class="op">);</span></span></code></pre></div>
<h2 id="signal-emission">Signal emission</h2>
<p>Signals are emitted on an instance. The type of the instance is the
second argument of <code>g_signal_new</code>. The relationship between
the signal and object type is determined when the signal is
registered.</p>
<p>A function <code>g_signal_emit</code> is used to emit the signal. The
following lines are extracted from <code>tfetextview.c</code>. Each line
comes from a different line.</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>g_signal_emit <span class="op">(</span>tv<span class="op">,</span> tfe_text_view_signals<span class="op">[</span>CHANGE_FILE<span class="op">],</span> <span class="dv">0</span><span class="op">);</span></span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>g_signal_emit <span class="op">(</span>tv<span class="op">,</span> tfe_text_view_signals<span class="op">[</span>OPEN_RESPONSE<span class="op">],</span> <span class="dv">0</span><span class="op">,</span> TFE_OPEN_RESPONSE_SUCCESS<span class="op">);</span></span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a>g_signal_emit <span class="op">(</span>tv<span class="op">,</span> tfe_text_view_signals<span class="op">[</span>OPEN_RESPONSE<span class="op">],</span> <span class="dv">0</span><span class="op">,</span> TFE_OPEN_RESPONSE_CANCEL<span class="op">);</span></span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a>g_signal_emit <span class="op">(</span>tv<span class="op">,</span> tfe_text_view_signals<span class="op">[</span>OPEN_RESPONSE<span class="op">],</span> <span class="dv">0</span><span class="op">,</span> TFE_OPEN_RESPONSE_ERROR<span class="op">);</span></span></code></pre></div>
<ul>
<li>The first argument is the instance on which the signal is
emitted.</li>
<li>The second argument is the signal id.</li>
<li>The third argument is the detail of the signal. “change-file” signal
and “open-response” signal doesn’t have details and the argument is zero
when no details.</li>
<li>“change-file” signal doesn’t have parameter, so there’s no fourth
parameter.</li>
<li>“open-response” signal has one parameter. The fourth parameter is
the parameter.</li>
</ul>
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